Water-injected screw compressor

ABSTRACT

The invention relates to a water-injected screw compressor which comprises a compressor element ( 1 ) with an interior space ( 3 ), a suction conduit ( 7 ) which connects to an inlet part ( 6 ), situated at the top of this interior space ( 3 ), a pressure conduit ( 9 ) in which a vessel ( 10 ) is installed, and an injection conduit ( 22 ) between the vessel ( 10 ) and the interior space ( 3 ), which comprises a part ( 31 ) which is situated higher than the upper side of the inlet part ( 6 ). A connection conduit ( 36 ) is provided between the inlet part ( 6 ), on one hand, and a connection point, situated above the maximum water level ( 35 ) of the vessel ( 10 ), onto the part ( 32 ) of this injection conduit ( 22 ) extending downward towards its exit, on the other hand.

[0001] This invention relates to a water-injected screw compressor whichcomprises a compressor element with two rotors driven by a motor, whichrotors are rotatable in an interior space of the compressor element, asuction conduit which connects to an inlet part, situated at the top ofthis interior space of the compressor element, with an inlet valve whichcan close off the exit giving out to this inlet part, a pressure conduitwhich connects to this interior space of the compressor element and inwhich a vessel, which at the same time is a water separator, isinstalled, whereby an injection conduit is installed between the vesseland the interior space of the compressor element, for the injection ofwater into this interior space, which injection conduit comprises a partwhich is situated higher than the upper side of the inlet part of theinterior space of the compressor element.

[0002] With most of the oil-injected compressors, at the outlet of thecompressor element or in the pressure conduit, a valve, mostly a returnvalve, is provided; a valve is also provided in the injection conduit.

[0003] The valve at the outlet prevents that, when the compressorelement suddenly stops before the inlet valve is closed, compressed airfrom the vessel flows outward through the compressor element and thesuction conduit.

[0004] The valve in the injection conduit prevents that, with a suddenstop, too much oil will flow through the injection conduit into theinterior space of the compressor element and this latter would be filledby oil, which subsequently would prevent the starting of the compressorelement, as oil is not compressible.

[0005] As the functioning of said valves, however, becomes unreliable ina watery environment, the valves at the outlet and in the injectionconduit of water-injected compressors mostly are omitted and a specialinlet valve is used in the suction conduit, to wit a so-called“unloader”.

[0006] This “unloader” is a controlled valve mechanism which closes offthe inlet just before or immediately after switching off the compressor,such that no compressed gas from the compressor element can be blowntowards outside through the suction conduit, and which prevents that, asa result of the pressure in the vessel, the compressor element should befilled with water through the injection conduit. Through this valvemechanism, after stopping, compressed gas can be blown off from thevessel.

[0007] When with such compressors, the inlet valve closes off thesuction conduit before or immediately after the stop of the compressorelement, the vessel and the compressor element are at the same pressure,in consideration of the fact that no return valve is present at theoutlet of the compressor element.

[0008] At that moment, the water level in the vessel is situated betweena minimum and a maximum, and if this water level is higher than theunderside of the compressor element, then, as no valve is present in theinjection conduit, water from the vessel will flow back to thecompressor element, as a result of the suction effect, until the waterlevel in the vessel is equal to the one in the compressor element.

[0009] The compressor element can become filled up with water in such amanner that starting becomes impossible.

[0010] In order to prevent this, the position of the vessel in respectto the compressor element is chosen such that the maximum water level inthe vessel is situated at the height of the bottom side of the rotors inthe compressor element.

[0011] This may cause problems for the construction of the compressorand has as a consequence that the compressor takes up relatively muchspace and therefore also the housing in which the vessel, the compressorelement and the motor are installed, is relatively large.

[0012] The invention aims at a compressor of the type without valves,neither at the outlet nor in the pressure conduit or in the injectionconduit, which does not have this disadvantage and can be realized in arelatively compact manner.

[0013] To this aim, according to the invention, a connection conduit isprovided between the inlet part of the interior space, on one hand, anda connection point, situated above the maximum water level of thevessel, onto the part of this injection conduit extending downwardtowards the lower-situated exit of the injection conduit in the interiorspace, on the other hand.

[0014] When a cooler is installed in the injection conduit, which cooleris situated higher than said maximum level, then, in order to prevent apossible draining of the cooler into the interior space, the cooler issituated in a central part of the injection conduit, which, at oppositesides of the cooler, locally rises up to above this cooler, whereas theparts of the injection conduit which connect to this central part andare directed upward, downward, respectively, are connected to each otherby means of a connection conduit, below the uppermost point of theinjection conduit.

[0015] With the intention of better showing the characteristics of theinvention, hereafter, as an example without any limitative character, apreferred form of embodiment is described, with reference to theaccompanying figure.

[0016] The water-injected screw compressor represented in this figurecomprises a compressor element 1 with a housing 2 which limits aninterior space 3 in which two cooperating screw-shaped rotors 4 arerotatably installed, which rotors are driven by a motor 5.

[0017] At one extremity, this interior space 3 is provided at the topwith an inlet part 6, into which a suction conduit 7 gives out by meansof an inlet valve 8.

[0018] At the other extremity, at the bottom, a pressure conduit 9connects to the interior space 3, with therein a vessel 10 which isprovided with a filter element 10A and which forms a water separator,and, downstream in respect to the vessel 10, a minimum pressure valve11.

[0019] The inlet valve 6 is a so-called “unloader” and substantiallyconsists of a cylinder-forming housing 12 which is provided with ahollow space 12A in which a piston 13 is displaceable. Between theoperative surface of the piston 13 and the housing, a cylinder chamber14 is formed. At the other side, the piston 13 is connected, by a pistonrod 15, to a valve element 16 which is situated in the inlet part 6 andcooperates with a valve seat 17 provided in the wall thereof.

[0020] A springy element in the form of a pressure spring 18 surroundsthe piston rod 15, between a part of the housing 12 and the piston 13,and pushes the piston 13 away and, therefore, the valve element 16towards or against the valve seat 17.

[0021] A control conduit 19 gives out into the cylinder chamber 14, atthe extremity opposed to the valve element 16.

[0022] The interior space 3 is in connection with the suction conduit 7or the interior of the inlet valve 8, upstream of the valve element 16,by means of a conduit 20, with therein a return valve 21 whichexclusively allows for a limited flow towards the interior space 3 andprevents the occurrence of a negative pressure in the interior space 3.

[0023] An injection conduit 22 connects the underside of the vessel 10to one or several injection points 23 which give out into the interiorspace 3.

[0024] In this injection conduit 22, starting from the vessel 10,successively a pump 24, a cooler 25 and a water filter 26 are installed.

[0025] The pump 24 is bypassed by a conduit 27 with therein a returnvalve 28 which exclusively allows for a flow from the vessel 10 towardsthe compressor element 1.

[0026] The cooler 25 is installed in a part of the injection conduit 22which is situated higher than the inlet part 6, and thus this injectionconduit 22, between the vessel 10 and the interior space 3,substantially consists of a part 29 situated below the vessel 10, anupward part 30 extending up to above the compressor element 1, a centralpart 31 with therein the cooler 25, a part 32 extending downward up tobelow the compressor element 1, and an end part 33 which connects to theinterior space 3.

[0027] The vessel 10 is provided with a level-measuring device whichrestricts the level of the water separated in the vessel 10 between aminimum level 34 and a maximum level 35 which is situated just below theinlet part 6.

[0028] Besides, this inlet part 6, just above this maximum level 35, isconnected, by means of a preferably horizontal connection conduit 36, tothe downwardly directed part 32 of the injection conduit 22, whereasthis part 32, just above the water filter 26 and thus above the inlet.part 6, however, below the highest point of the injection part 22 and inthe represented example even below the cooler 25, is connected to theupwardly directed part 30 of the injection conduit 22 by means of apreferably horizontal connection conduit 37.

[0029] When the water-injected screw compressor 1 is switched on, themotor 5 drives the rotors 4. Thereby, air is suctioned through suctionconduit 7 and the inlet valve 8 is opened.

[0030] In the interior space 3, the suctioned air is compressed, andsubsequently the compressed air is fed through the pressure conduit 9towards the vessel 10, where the water is separated from the air.

[0031] During this working, water is injected through the injectionconduit 22 into the injection points 23 of the compressor element 1, inorder to lubricate and cool the various bearings and the rotors 4.Thereby, this water is cooled in the cooler 25 and filtered in the waterfilter 26.

[0032] The minimum section of the connection conduit 37 is relativelysmall in respect to the section of the part 32 of the injection conduit22, and thus also of the part 30 having the same section, such that onlya small portion of the water does not flow through the cooler 25.

[0033] The connection conduit 36 has a minimum section, for example, theexit to the inlet part 6, which clearly is smaller than the section ofthe part 32 of the injection conduit 22, such that also only a tinyportion, and preferably less than 5%, of the water is branched off byconnection conduit 36 towards the inlet part 6.

[0034] As, when starting the compressor, the pressure in the vessel 10is not high enough, first the pump 24 is also switched on for waterinjection. As soon as the pressure in the vessel 10 is high enough, thepump 24 is switched off and the water is pressed, by the return valve28, through the injection conduit 22 as a result of the pressure in thevessel 10.

[0035] As soon as the pressure in the vessel 10 surpasses a pressure,which has to be pre-set, of the minimum pressure vessel 11, this latteropens, and compressed air is delivered.

[0036] When the compressor is stopped, the inlet valve 8 is closed as aresult of the drop of the control pressure through the control conduit19.

[0037] At that moment, the water level in the vessel 10 is between theminimum level 34 and the maximum level 35, whereas the pressure in thisvessel 10 is somewhat higher than the pressure set by the minimumpressure valve 11 and which is equal to the pressure at the outlet ofthe interior space 3. This pressure first will drop by means of theminimum pressure valve 11, and then by means of a blow-off mechanism,not represented in the figure, which is integrated into the the specialinlet valve 8 constructed as a so-called “unloader”.

[0038] If the connection conduit 36 should not be present, due to thesuction effect the water level in the vessel 10 and in the interiorspace 3 should get to an equal level, as the pressure in both spaces isequal.

[0039] Due to this connection conduit 36, two pairs of communicatingvessels are created, a first pair with the vessel 10 and the interiorspace 3, which are connected to each other by the connection conduit 36and the part of the injection conduit 22, situated upstream thereof,comprising the cooler 25, and a second pair of communicating vesselsformed by an inlet part 6 and the interior space 3, which are connectedto each other by the connection conduit 36 and the part, situateddownstream thereof, which comprises the part 33.

[0040] Both pairs are trying to find an equilibrium, and air will besuctioned, through connection conduit 36, into the lowermost part of thepart 32 of the injection conduit 22, until the level in this part 32drops up to approximately the bottom side of the interior space 3.

[0041] Also, the uppermost portion of the part 32 of the injectionconduit 22 can partially drain into the compressor element 1, as thepart 32 extends upward towards the cooler 25, up to the highest point ofthis part 32.

[0042] The highest point, where it connects to the central part 31, issituated above the cooler 25 in order to prevent a draining of thelatter. At opposite sides of this cooler 25, the central part 31comprises a part situated higher than the cooler 25.

[0043] As a result of the suction effect, the cooler 25 still mightdrain towards the compressor element 1, by means of thenot-selfsuctioning pump, if the second connection conduit 37 were notpresent.

[0044] Due to this connection conduit 37, an air bubble is created inthe part 30 of the injection conduit 22, and the water level in thispart 30 becomes situated at the same height as the water level in thevessel 10. Above this level, the part 30 is drained.

[0045] Due to the connection conduits 36 and 37, the vessel 10 can besituated with its maximum level 35 well above the bottom side of theinterior space 3, and, as a result, it can be installed next to thecompressor element 1, in the position most advantageous for theconstruction of the compressor.

[0046] In a variant, the connection conduit 36, the final part 33 of theinjection conduit 22 and the part, situated below the connnectionconduit 36, of the part 32 of the injection conduit 22 can be integratedin the housing 2 of the compressor element 1.

[0047] The present invention is in no way limited to the form ofembodiment described as an example and represented in the figures,however, such water-injected screw compressor can be realized indifferent forms and dimensions, without leaving the scope of theinvention.

1.- Water-injected screw compressor which comprises a compressor element(1) with two rotors (4) driven by a motor (5), which rotors arerotatable in an interior space (3) of the compressor element (1), asuction conduit (7) which connects to an inlet part (6), situated at thetop, of this interior space (3) of the compressor element (1), with aninlet valve (8) which can close off the exit giving out onto this inletpart (6), a pressure conduit (9) which connects to this interior space(3) of the compressor element (1) and in which a vessel (10), which atthe same time is a water separator, is installed, whereby an injectionconduit (22) is installed between the vessel (10) and the interior space(3) of the compressor element (1), for the injection of water into thisinterior space (3), which injection conduit (22) comprises a part (31)which is situated higher than the upper side of the inlet part (6) ofthe interior space (3) of the compressor element (1), characterized inthat a connection conduit (36) is provided between the inlet part (6) ofthis interior space (3), on one hand, and a connection point, situatedabove the maximum water level (35) of the vessel (10), onto the part(32) of this injection conduit (22) extending downward towards thelower-situated exit of this injection conduit (22) in the interior space(3), on the other hand. 2.- Water-injected screw compressor according toclaim 1, characterized in that the connection conduit (36) has a smallermaximum section than the section of the downwardly extending part (32)of the injection conduit (22). 3.- Water-injected screw compressoraccording to claim 1 or 2, characterized in that the connection conduit(36) extends approximately horizontal. 4.- Water-injected screwcompressor according to any of the preceding claims, characterized inthat it comprises a cooler (25) in the injection conduit (22) which issituated higher than the aforementioned maximum water level (35) and, inorder to prevent a possible draining of the cooler (25) towards theinterior space, the cooler (25) is situated in a central part (31) ofthe injection conduit (22), which, at opposite sides of the cooler (25),locally rises up to above this cooler (25), whereas the parts (30,32) ofthe injection conduit (22) which connect to this central part (31) andare directed upward, downward, respectively, are connected to each otherby means of a connection conduit (37), below, the uppermost point of theinjection conduit (22). 5.- Water-injected screw compressor according toclaim 4, characterized in that the connection conduit (37) between theparts (30,32), extending upward, downward, respectively, of theinjection conduit (22) is situated above the maximum level (35). 6.-Water-injected screw compressor according to claim 5, characterized inthat the connection conduit (37) between the parts (30,32) of theinjection conduit (22) has a minimum section which is smaller than theminimum section of the portions of the parts (30,32) to which theconnection conduit (37) connects. 7.- Water-injected screw compressoraccording to claim 5 or 6, characterized in that the connection conduit(37) between the parts (30,32) extends approximately horizontal and issituated above the connection conduit (36).